The technical field of this invention is power assist in vehicle braking systems.
A well known power assist apparatus for a fluid operated vehicle braking system inserts a vacuum brake booster between the operator input pedal and the master cylinder. The booster includes a mechanical through path to the master cylinder for braking force exerted by the vehicle operator but additionally includes a valve controlled vacuum actuator responsive to pedal position for augmenting the operator exerted force. The assist force generating mechanism is a differential pressure across a diaphragm between a pair of chambers, one of which is provided with air at lower than atmospheric pressure, generally from engine vacuum, and the other of which is provided by a controlled valve apparatus with air at a working pressure that increases from the engine vacuum level of the one chamber generally proportionally with vehicle operator input braking force through the brake pedal. As the operator input braking force increases, the differential pressure across the diaphragm likewise increases, and so does the braking assist force applied to the master cylinder.
Such systems are subject to a well known limitation in brake assist force known as runout. The source of operating pressure for the booster, whether derived from engine vacuum or from any other source of vacuum, pressure chamber of the booster reaches atmospheric pressure. When the booster is in a runout condition, additional operator input braking force, although it is itself transmitted through the booster to the master cylinder, produces no further increase in assist pressure, since atmospheric pressure is the highest pressure in the system. A simplified illustration of the. result is shown in FIG. 4, which plots the output force of the booster as a function of the input force from the vehicle operator. The total output force, which is the sum of the input force and the assist force generated by the booster, increases generally proportionally with the input force as shown in plot 100 until runout is achieved at point 104. Above this point, in the runout condition, the booster produces no additional assist force; and the total force thus increases at a lower rate due only to the increase in input force, as seen in plot 102. Since this change in the rate of increase in braking force is easily felt by the operator through the brake pedal, such brake boosters are generally designed so that runout is not reached during normal driving conditions; and a smooth, consistent braking feel is provided in brake operation. But vehicle designs are tending to produce smaller engine compartments, due to an increase in underhood equipment together with increased streamlining for improved fuel economy. Vacuum brake boosters tend to be bulky; and this bulkiness is aggravated by a need for the greater output force capacity needed for a higher runout point. A smaller booster will save valuable space in the engine compartment, if the booster output can be supplemented in runout.
Various ways of supplementing booster output in runout have been suggested. Most of these provide additional capacity to the booster through a higher vacuum level to the vacuum chamber or a positive pressure above atmospheric to the working chamber and thus involve an additional pump and other apparatus. This invention, however, has to do with an approach using equipment already on many, if not most, vehicles: the anti-lock braking system (ABS). U.S. Pat. No. 5,938,297 to Whaite et al, Method and Device for Brake Application, issued Aug. 17, 1999, describes the use of ABS/TCS brake fluid pump and modulators included in many vehicles for providing anti-lock braking and brake-based traction control (TCS) to provide supplementary braking pressure at the wheels during booster runout conditions. The system of the patent is disclosed as being responsive to a signal of a runout condition and an additional input force beyond that producing the runout condition to operate the ABS pump and modulator valves to provide additional brake fluid pressure at the wheel brakes to supplement that from the master cylinder so as to attempt to maintain a substantially linear relationship between input braking force from the vehicle operator and actual braking force at the wheel brakes when the vacuum booster is in runout.
But the system as described in the patent is not without potential for improvement. The coordination of braking force between the booster and the brake modulator valves to provide a total force increasing in a smooth linear fashion as the booster reaches runout has turned out to be a difficult matter. A very simple control algorithm based on the booster force model of FIG. 1 could, when booster vacuum sensor 33 signals no booster vacuum in the working chamber, multiply a signal of input braking force from the pedal force sensor 32 by a system gain constant to derive a target wheel brake pressure and subtract therefrom the master cylinder pressure from sensor 31. The difference would be the required additional brake fluid pressure to be provided by the modulator 20. But it turns out that, although such a simple control algorithm generally works to supplement the booster and provides substantially linear control once the booster is well into a runout condition, the onset of runout is not as clearly defined as the simple model implies. There is a region of operation just below the point at which booster vacuum reaches zero in which the response of the system loses linearity, quite noticeably. The result can be a brake xe2x80x9cfeelxe2x80x9d that is unpleasant to a vehicle operator, and therefore undesirable. In addition, a pedal force sensor may not be available for a control as described above.
The invention is a method for providing a substantially constant brake assist gain in a vehicle braking system having a vacuum booster providing braking assist to a brake fluid master cylinder in response to an operator input braking force. The vacuum booster provides a booster gain having a maximum value equal to the substantially constant brake assist gain over a substantial portion of its working vacuum range but falling from the maximum value to a unitary value in a non-linear variation over a runout initiating portion of the working vacuum range. The method a brake fluid pump, senses booster vacuum and, responsive to a sensed booster vacuum in the runout initiating portion of the working vacuum range,
(1) derives a runout boost assist gain from stored data expressing booster assist gain as a function of booster vacuum;
(2) derives a target wheel pressure from the derived runout boost assist gain and a corresponding change in master cylinder pressure;
(3) determines a target wheel pressure error from the difference between the derived target wheel pressure and the master cylinder pressure;
(4) activates the brake fluid pump and routing brake fluid from the master cylinder thereto; and
(5) provides means for controlling pressure of fluid from the brake fluid pump and controls said pressure in response to the target wheel pressure error.
In an embodiment of the invention, the stored data may comprise a piecewise linear approximation of the non-linear variation of booster assist gain as a function of booster vacuum in the runout initiating portion of the working vacuum range and the change in master cylinder pressure used in the derivation of target wheel pressure may be the difference between a currently sensed value of master cylinder pressure corresponding to a currently derived runout boost assist gain and an immediately preceding sensed value of master cylinder pressure.
The method of this invention overcomes the shortcoming of the prior art based on the discovery that the simple plot of booster output versus input force shown in FIG. 4 is not completely accurate in the vicinity of the onset of runout. A more accurate plot of vacuum booster output force as a function of input force appears in FIG. 5. This plot illustrates that runout does not occur suddenly, at a single point, when the working chamber pressure reaches atmospheric pressure. Rather, the output/input force slope starts to roll off somewhat below this point: e.g. at a vacuum level of about 2.5 inches; and the onset of runout is process spanning a range of booster vacuum adjacent the zero vacuum level. It is this roll-off which produces the non-linearity of a control algorithm based on the simplified model of FIG. 4. In addition, the linear increase in booster pressure due to additional driver input braking force in runout produces an output plot offset somewhat downward from the plot predicted by the simpler model of FIG. 4. The control method of this invention is based on the more accurate plot of FIG. 5 and is thus able to maintain linearity through the onset of runout and an output braking force plot in runout co-linear with that below runout. In addition, the method of this invention does not require a brake pedal force sensor and is thus adaptable to many more vehicles.